1. Field of the Invention
The present invention relates to a method and apparatus for transmitting sound through and reflecting sound from a solid object. More specifically, the invention relates to a method and apparatus for altering the characteristics of an object such that sound waves will alternatively pass completely therethrough or be completely reflected therefrom.
2. Description of the Prior Art
A general characteristic of solid objects is that they reflect sound waves. Sound is transmitted in waves comprised of compressions and rarefactions of the medium through which the sound is passing, be it a solid, liquid or gas. The transmission characteristics of each medium are based on the density of the medium. Gas, being the most compressible of the three, is the poorest conductor of sound waves. Solids, therefore, provide the best transmission of sound. Solids, furthermore, due to their inherent formability, may be adapted to transmit sound wave energy over a distance in a controlled manner.
This characteristic has been utilized to create the acoustic waveguide. The acoustic waveguide is analogous to a wire conductor in the electrical field. Just as the wire conductor permits the controlled passage of a current from one end to another, the acoustic waveguide transmits sound wave energy from one point to another. The waveguide is generally in the form of an elongated solid, similar to a thick wire. The waveguide may also be hollow or in any geometric form. Additionally, the transmission medium within the waveguide may be a contained liquid or gaseous material. A common example of a waveguide is the medical stethoscope.
The waveguide, in its preferred embodiment, is tuned in shape and density to the frequency of the sound wave that is to be carried Such tuning allows for the optimal transmission of sound energy within the waveguide. A waveguide which is intended to carry a range of frequencies, however, must be adapted to carry all at some acceptable level, rather than any one at its optimal level.
Sound wave energy is also affected by changes in the transmission medium. When sound waves encounter a change in the density of the transmission medium, at least a portion of the wave energy is reflected away from the surface of the new medium. The remaining portion passes through this medium. Acoustic impedance, which is the product of the density of the waveguide material and the velocity of sound in the material, is utilized to describe and measure the reflection of sound waves at the interface of two media. If the acoustic impedances of two abutting materials are radically different, then reflection of sound waves within one medium from the interface is maximized. As the acoustic impedances of the two materials become closer in value, reflection is lessened, and a greater portion of the sound wave energy passes into and through the second medium. If the acoustic impedances are equal, the sound wave energy will pass with little or no loss into and through the second medium.
Sound waves will also pass through objects which have a thickness equal to one half of their wavelength. This is regardless of the change in acoustic impedance between the object and its surrounding medium. This, like an interface between media of equal acoustic impedance, will cause little or no reflection of the sound waves.
The limitation of this phenomena is that the object must be sized exactly one half the wavelength of the sound which is intended to pass therethrough. If the wavelength of the sound waves is unknown, or of a varying quantity, then the required size of the object cannot be predicted.
What has not been recognized by the art, therefore, is a method or apparatus which can change the perceived thickness of an object, such that a sound wave having a wavelength which is not preselected will pass therethrough without substantial or detectable reflection, or alternatively, will be completely reflected therefrom.